1. Field of the Invention
The present invention relates to electrostatic discharge protection method and device for a semiconductor device. In particular, the invention relates to electrostatic discharge protection method and device for a semiconductor device with an electrostatic discharge protection element that functions as a discharge path of a surge current generated due to static charges impressed to an external terminal of the semiconductor device.
2. Description of Related Art
If static charges are impressed to a terminal of a semiconductor device, an abnormal current (hereinafter referred to as “surge current”) flows through a circuit due to the static charges. If the surge current is generated, there arises a problem that a terminal voltage increases and an internal circuit is broken down, for example. The breakdown resulting from the electrostatic charges is hereinafter referred to as “electrostatic breakdown”.
To that end, general semiconductor devices incorporate an electrostatic discharge protection device for the purpose of securing a surge current discharge path and preventing a terminal voltage from increasing. Japanese Unexamined Patent Application Publication No. 5-267588 discloses an example of an electrostatic discharge protection device.
FIG. 24 is a circuit diagram of an electrostatic discharge protection device 100 of the above related art. As shown in FIG. 24, the electrostatic discharge protection device 100 of the related art includes an NPN transistor 101, a PNP transistor 102, diodes 103 to 105, an input/output terminal (I/O terminal), a power supply terminal (VDD terminal), and a ground terminal (GND terminal). The I/O terminal is an input/output terminal of the semiconductor device. The I/O terminal is connected with an internal circuit. The NPN transistor 101 has a collector connected with a VDD terminal, an emitter connected with an I/O terminal, and a base connected with a GND terminal. The PNP transistor 102 has a collector connected with the GND terminal, an emitter connected with the I/O terminal, and a base connected with the VDD terminal.
The diode 103 has an anode connected with the GND terminal and a cathode connected with the VDD terminal. The diode 104 has an anode connected with the I/O terminal and a cathode connected with the VDD terminal. The diode 105 has an anode connected with the GND terminal and a cathode connected with the I/O terminal.
Description is made of how the electrostatic discharge protection device 100 protects an internal circuit against breakdown. Electrostatic charges are impressed to the I/O terminal based on a potential level of the VDD terminal or the GND terminal (reference potential). Further, as the impressed electrostatic charges, there are positive charges and negative charges. First, if positive electrostatic charges are impressed based on the potential level of the VDD terminal (VDD+ impressed), the diode 104 is reverse-biased, or the diode 105 is broken down and the diode 103 is reverse-biased. As a result, a surge current flows into the VDD terminal. On the other hand, if negative electrostatic charges are impressed based on the potential level of the VDD terminal (VDD− impressed), the breakdown of the diode 104 or the diode 103 occurs, and the diode 105 is forward-biased. Alternatively, the NPN transistor 101 is turned on due to a charging current to a parasitic capacitance of the diode 103 on the rising edge of a surge current. As a result, a negative surge current flows into the VDD terminal.
Next, if positive electrostatic charges are impressed based on the potential level of the GND terminal (GND+ impressed), the breakdown of the diode 105 or diode 103 occurs, and the diode 104 is reverse-biased. Alternatively, the PNP transistor 102 is turned on due to a charging current to a parasitic capacitance of the diode 103 on the rising edge of a surge current. As a result, a surge current flows into the GND terminal. On the other hand, if negative electrostatic charges are impressed based on the potential level of the GND terminal (GND− impressed), the diode 105 is forward-biased, or the diode 103 is forward-biased and the breakdown of the diode 104 occurs, with the result that a negative surge current flows into the GND terminal.
The electrostatic discharge protection device 100 protects the internal circuit from breakdown by discharging the surge current through the above path. Further, the electrostatic discharge protection device 100 has a trench-like insulative region (trench oxide film) around a region where a terminal connected with the I/O terminal is formed among the terminals of the element. The trench oxide film decentralizes an electric field generated at the edge of a region where the terminal connected to the I/O terminal is formed, if a surge current flows. As a result, a breakdown voltage of components of the electrostatic discharge protection circuit is increased to protect the components from breakdown if the surge current is impressed.
However, in the electrostatic discharge protection device 100 of the related art, a surge current is discharged through the broken-down path. A breakdown voltage at which the breakdown of the diode occurs is generally high. Thus, if the internal circuit is protected with the discharge path passing through the broken-down diode, the terminal voltage is higher than a breakdown voltage of the internal circuit, and the internal circuit cannot be adequately protected.
Further, an amount of current that can flow through the broken-down diode is smaller than that of the forward-biased diode. Therefore, in the case of discharging a larger amount of surge current, a diode area is increased. Further, in electrostatic discharge protection device 100 of the related art, a trench oxide film is used for increasing a breakdown voltage of the diode, so an additional area is necessary for forming the trench oxide film. That is, the formation of the trench oxide film leads to an increase in area of the electrostatic discharge protection device 100. Moreover, if the surge current is discharged through the broken-down diode, the diode is deteriorated or broken down. The electrostatic discharge protection device needs to discharge a surge current to protect an internal circuit and protect circuit components against breakdown. If the circuit components are broken down, the following failure occurs. That is, a terminal potential is fixed to a potential of the VDD terminal or a leak current increases, for example.